The present invention concerns a continuously variable electromagnetic transmission, including a commutator-less, axial flux dynamoelectric machine provided with an input shaft and an output shaft, and means for controlling and supplying electric power at a variable frequency to said machine, the dynamoelectric machine including a first rotor connected to the input shaft, a second rotor connected to the output shaft and a stator assembly, the two rotors and the stator assembly being formed by discoid elements, the stator and at least one of the rotors having respective active elements provided with windings connected to the control and supply means and arranged to interact with the other rotor by means of magnetic flux through air gaps including axial air gaps between respective discoid elements of the rotors and the stator assembly.
Continuously variable electromagnetic transmissions comprising two rotors which thus co-operate with a common stator have already been proposed prior to 1920 in the form of-commutator machines, in particular direct current machines. See for example U.S. Pat. Nos. 1,392,349 and 1,515,322, which disclose axial flux machines, and U.S. Pat. No. 1,493,853 which discloses a radial flux machine with two concentric rotors inside a single stator. However, these commutator machines have a complicated construction and require a lot of maintenance, so that they have not met with commercial success.
Since several years, there has been a resurgence of interest in electromagnetic transmissions of this type, because progress in power electronics has enabled them to be powered at a variable frequency, determined as a function of the respective speeds of the shafts and as a function of the desired power flux, and has enable them to be made in the form of commutator-less machines, for example synchronous machines with permanent magnets.
Patent Application Nos. EP 0 798 844 and WO 99/39426 (corresponding to U.S. Pat. No. 6,373,160) disclose continuously variable transmissions of the type indicated in the preamble, of the axial field type, i.e. the rotor and stator assembly elements have a generally discoid shape. The use of discoid elements offers in particular easy access to each shaft from the stator, which enables the electric power connections to be easily made on rotating rings secured to the shaft, and a cooling fluid to be distributed into the heart of the machine. Moreover, the discoid shape of the elements enables great freedom as to the dimensions and arrangement of the elements, for example by grouping several modular elements on a same shaft in order to increase the power of the machine or by using elements of different diameters or of different types in the same machine.
The object of the present invention is to improve an electromagnetic transmission with axial flux and discoid elements so as to improve its operating conditions and thus broaden its possible uses.
The invention therefore concerns an electromagnetic transmission of the type indicated in the preamble, characterised in that it includes displacement means for axially displacing at least one of the discoid elements to modify the width of the axial air gap between this element and an adjacent discoid element.
The possibility of varying the width of an air gap, via axial displacement of a discoid element, allows the magnetic field to be adjusted as a function of the desired operating conditions, which enlarges the speed range and allows the working of the machine to be optimised. This is practically impossible with rotors of cylindrical shape. Moreover, via axial displacement of one or more elements, the discoid shape of the rotor elements allows direct mechanical coupling between the rotors so as to establish, upon demand, direct meshing between the input and output shafts, without having to add the separate clutch provided in European document No. 0 866 544 in the case of a machine of cylindrical shape. This coupling may advantageously use the displacement means intended for modifying the width of an air gap.